The present invention relates in general to heating, ventilating and air-conditioning (HVAC) and, more particularly, to control of a fan powered terminal unit wherein the fan air flow or cubic feet per minute (CFM) can be preset without regard for the installation specifics of the HVAC system in which the terminal unit is to be used. While the fan control will be described with reference to a fan powered terminal unit for which the present invention is initially being applied, it is to be understood that the fan control arrangement of the present application may be used in a variety of HVAC applications including, for example, venting hoods, exhaust fans, supply fans and fan coils.
Fan-powered terminal units which provide variable air volume (VAV) are used for both cooling and heating of perimeter zones of a building. The terminal units use the free heat derived from lighting, people and other equipment within the building by inducing this warmer air from a building core ceiling plenum space and recirculating it to rooms calling for heat. If additional heat is required, supplemental heating coils within the terminal units may be activated thus eliminating the need for a central source of warm air.
Once a HVAC system is installed, it must be set-up by setting the terminal units to provide required air flow for the system. A portion of this set-up currently includes placing a flow measuring hood over an air outlet or diffuser and going into the ceiling to adjust an associated VAV terminal unit to obtain the proper air flow. This portion of the HVAC system set-up adds substantially to the cost with labor related costs currently running around $40 to $60 dollars per terminal unit.
An electronically commutated motor (ECM) is commercially available from General Electric which can provide a fixed air flow for a terminal unit regardless of static pressure provided the static pressures are within a permissible range. The ECM motor is controlled using a pulse width modulation (PWM) control signal with the pulse width being proportional to the air flow which is to be delivered by the terminal unit. Unfortunately, the width of the pulses of the PWM signal can not be readily determined without equipment which is not normally available to manufacturing personnel and HVAC contractors. Accordingly, while the ECM motors offer the promise of being able to preset the air flow for terminal units on the manufacturing floor before shipment and also once installed to accommodate changing conditions of a HVAC system, terminal units currently using ECM motors must still undergo the high cost air flow balancing procedures.
Accordingly, there is a need for an improved control arrangement for fans used in fan-powered terminal units which permit the air flow of the terminal units to be factory preset and reset without the conventional labor intensive balancing procedures currently required. Preferably, the fan control arrangement could be used for a variety of HVAC applications such as venting hoods, exhaust fans, supply fans and fan coils.